Prepaid cards used for an automatic cash dispenser, a telephone, an automatic ticket publisher, etc. or papers, films and the like used for a printer, a facsimile, a copier, etc. (referred to as "sheet-type objects" hereinafter) are conventionally conveyed by a ultrasonic motor which conveys the sheet-type objects by resonating a prism vibrator made of a metal prism in flexural vibration with its resonance frequency, thereby to rotate cup-shaped rollers provided at both opposite ends of the prism vibrator (refer to Japanese Patent Laid-Open Publication Nos. 1-274674 and 1-274675).
FIG. 2 is a perspective view of a piezoelectric vibrator used by a conventional ultrasonic motor.
In such figure, denoted at 100 is a piezoelectric vibrator, which comprises a metal prism 101 having a substantially square-shaped cross section and piezoelectric ceramic thin plates 102a and 102b bonded to two adjacent side surfaces of the piezoelectric vibrator 100. The piezoelectric ceramic thin plates 102a and 102b are polarized in the direction of thickness and electrodes, not shown, are formed on the outer facing and back surfaces thereof. When an alternating voltage is applied to the piezoelectric ceramic thin plates 102a and 102b from the electrodes according to a predetermined method, an electric field is generated to make the piezoelectric ceramic thin plates 102a and 102b expand and contract to vibrate. In this case, since the metal prism 101 has substantially a square cross section, it effects flexural vibrations having resonance frequencies substantially equal to each other in directions perpendicular to each other.
When alternating voltages the frequencies of which are equal to the resonance frequency of the metal prism 101 and which are different in phase from each other by 90.degree. are applied to the piezoelectric ceramic thin plates 102a and 102b, both ends of the metal prism 101 effect rotating vibration or elliptical vibrations. The metal prism 101 is equipped with discs 103a and 103b at opposite ends thereof and supporting pins 104a and 104b at the nodes of flexural vibration generated therein to be stably supported thereby.
FIG. 3 is a perspective view of a conventional ultrasonic motor.
In such figure, denoted at 100 is a piezoelectric vibrator, 101 is a metal prism, 103b is a disc and 104a and 104b are supporting pins.
Cup-shaped rollers 201a and 201b each having an inner diameter slightly larger than the outer diameter of discs 103a and 103b are provided on the outer periphery of discs 103a (FIG. 2) and 103b mounted on opposite ends of the metal prism 101. As a result, the rotating or elliptical vibrations of the opposite ends of the metal prism 101 bring the discs 103a and 103b into contact with the cup-shaped rollers 201a and 201b to rotate the same by friction. The cup-shaped rollers 201a and 201b are rotatably supported by bearings 202a and 202b respectively.
FIG. 4 is a perspective view of a conventional ultrasonic transfer device.
In such figure, denoted at 100 is a piezoelectric vibrator, 101 is a metal prism, 104b are supporting pins, 201a and 201b are cup-shaped rollers and 202a and 202b are bearings.
Auxiliary rollers 301a and 301b which are rotatably supported by bearings 302a and 302b are provided in such a way as to press on the cup-shaped rollers 201a and 201b. The auxiliary rollers 301a and 301b are connected to each other by way of a shaft 303.
The rotating or elliptic vibrations at the opposite ends of the metal prism 101 rotates the cup-shaped rollers 201a and 201b thereby to rotate the auxiliary rollers 301a and 301b following thereto. As a result, if a sheet-type object, not shown, is inserted between the cup-shaped rollers 201a and 201b and the auxiliary rollers 301a and 301b, such object can be easily conveyed.
The conventional ultrasonic motor and ultrasonic conveying device, however, cannot receive a force applied to the metal prism 101 by the expanding and contracting vibration of the piezoelectric ceramic thin plates 102a and 102b. Thus, a force applied to the metal prism 101 as the cup-shaped rollers 201a and 201b press on the discs 103a and 103b does not conform in direction to that applied to the metal prism 101 by the expansion and contraction of the piezoelectric ceramic thin plates 102a and 102b during conveyance of the sheet-type objects. Accordingly, when the cup-shaped rollers 201a and 201b press the discs 103a and 103b more strongly, they influence the bending rigidity of the metal prism 101 and bonding strength between the metal prism 101 and the piezoelectric ceramic thin plates 102a and 102b. This causes fluctuation in resonance frequency and amplitude of rotating and elliptical vibrations, resulting in failure to generate a larger torque.
Moreover, since the metal prism 101 and cup-shaped rollers 201a and 201b are individually supported, it is difficult to correctly position the ultrasonic motor and ultrasonic conveying device relative to each other in mounting the same to various devices. This can cause uneven rotation of the cup-shaped rollers 201a and 201b.
It is the object of the present invention to solve the problems of the conventional ultrasonic motor and ultrasonic conveying device set forth above and to provide an ultrasonic motor and an ultrasonic conveying device capable of generating larger torque and stabilizing the rotation thereof.